The present invention relates to the production of atactic or amorphous poly-alpha-olefins. More particularly it relates to a high temperature process for the production of non-crystalline or low crystallinity polymers and copolymers.
In the early processes for the production of isotactic or crystalline polypropylene, the initial catalyst systems (brown or violet titanium trichloride) were not very stereospecific and appreciable amounts of amorphous or atactic polypropylene were coproduced along with the desired isotactic polymer. These atactic or amorphous polymers were low melting non-crystalline or low crystallinity polymers with poor physical properties compared to isotactic polypropylene. The atactic or amorphous polymer had to be removed by solvent extraction so that the properties of the isotactic polymer were not degraded. At first, this material was burned or discarded by burial. Gradually, means were found to purify this material by removing the catalyst residues and solvent, and the resulting tacky, low molecular weight solid material found applications as an ingredient in hot melt adhesives, sealants, and bituminous formulations. However, the product quality was variable, with no two batches of material being exactly the same. The situation was further complicated by the fact that some material resulted from copolymer production and other material resulted from homopolymer production. The market for the amorphous material developed nicely. However, as the catalysts were improved, the amorphous polymer content was reduced to such a low value that it was not necessary to remove the atactic or amorphous polymer from the isotactic polypropylene. Producers found the demand for atactic or amorphous polymers outstripping the supply. Material which had been discarded earlier was mined from atactic pits and purified by acid washing and steam distillation. One polypropylene producer's catalyst system for isotactic polymer was very poor and large quantities of atactic were produced, giving that producer a reasonable supply of amorphous poly-alpha-olefins (APAO). Another producer converted an isotactic polypropylene line to amorphous production and used a magnesium chloride (MgCl2) supported titanium catalyst minus the stereoregulator to produce “on purpose” APAO. This approach increased the quality and consistency of the amorphous product and allowed production of copolymers and terpolymers which had properties which were appreciably different from the coproduct atactic produced with isotactic polypropylene. This approach also opened up a potential for different materials which might have new applications.
U.S. Pat. No. 4,736,002 (issued Apr. 5, 1988 to G. Allen et al.) describes a process for preparing amorphous polypropylene and copolymers at relatively low temperatures of 150° F. in liquid propylene, utilizing a converted isotactic polypropylene polymerization line. The disadvantages of this process are the necessity to operate the polymerization at low temperatures due to the inherent tackiness of the amorphous polymers and copolymers and the resulting low polymerization rates.
As a result of the building of new plants using the improved catalysts and the closing of old plants, there has occurred a shortage of atactic polypropylene.
It is an object of the present invention to provide a process to produce predominately atactic polypropylene for use in the roofing, adhesive and other markets. It is also an object of this invention is to provide a high temperature polymerization or copolymerization process in a tubular reactor to maximize the polymerization rate and reduce the stereoregularity of the polymer.
Another object of the present invention is to produce amorphous alpha-olefin polymers and copolymers which can be used in many applications. It is also an object of the present invention to prepare new polymers and copolymers which can not be produced by existing processes for preparing amorphous polyolefins.